1. Field of the Invention
The present invention relates generally to hearing prostheses and, more particularly, to cochlear implant systems.
2. Related Art
The use of patient-worn and implantable medical devices to provide therapy to individuals for various medical conditions has become more widespread as the advantages and benefits such devices provide become more widely appreciated and accepted throughout the population. In particular, devices such as hearing aids, implantable pacemakers, defibrillators, functional electrical stimulation devices such as cochlear™ implant systems, organ assist or replacement devices, and other medical devices, have been successful in performing life saving and/or lifestyle enhancement functions for a number of individuals.
One category of such medical devices is hearing prostheses which include but are not limited to hearing aids and cochlear™ implant systems. Hearing aids are externally-worn devices which amplify sound to assist recipients who have degraded or impaired hearing due to, for example, age, injury or chronic ear or mastoid infections. Cochlear™ implant systems provide the benefit of hearing to individuals suffering from severe to profound hearing loss. Hearing loss in such individuals is due to the absence or destruction of the hair cells in the cochlea which transduce acoustic signals into nerve impulses. Cochlear™ implant systems essentially simulate the cochlear hair cells by directly delivering electrical stimulation to the auditory nerve fibers. This causes the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to auditory nerves.
Conventional cochlear™ implant systems have generally included an external assembly directly or indirectly attached to the body of the patient (sometimes referred to herein as the recipient), and an internal assembly which is implanted in the patient. More recently, some cochlear™ implant systems have been designed such that all of the systems' components are implanted subcutaneously; that is there is no external assembly. Because such cochlear™ implant systems are entirely implantable, they are commonly referred to as a “totally” implantable cochlear™ implant.
Cochlear™ implant systems generally comprise a microphone for detecting sounds and a speech processor that converts the detected sounds into a coded signal provided to a stimulator unit which drives an electrode array implanted in the cochlea of the patient. The coded signals are applied by the electrode array to the basilar membrane thereby stimulating the auditory nerve.
A cochlear implant speech processor should be able to calculate an ideal neural excitation pattern, and then produce that pattern in the auditory neurons by activating the electrodes with appropriate electric current levels or charges per pulse. Specifically, the objective of pulsatile electrical encoding for cochlear implants is to convert an ideal spatio-temporal pattern of excitation (referred to herein as an ideal excitation pattern) to a sequence of variable biphasic stimulus pulses (referred to herein as a stimulation signal) that will best create the same pattern when presented on the implanted electrodes. The exact nature of excitation and its distribution in the cochlear and auditory pathway is open to debate, but the generally-accepted approach to quantify excitation is loudness.
The essential requirement of the encoding task is to recreate the ideal excitation pattern with as little error and reduction of information as possible, while minimizing perceptual artifacts from the pulsatile nature of the stimulation. With recent interest in patterns of excitation with greater temporal information, the effects of summation of loudness, and in the importance of technical improvements such as higher rates & more electrodes, it is useful to consider current encoding methods, and make improvements where possible.